The present invention relates to operational amplifiers and other mixed signal circuits. More particularly, the present invention relates to an integrated current mirror circuit and technique for facilitating improved rail-to-rail performance in operational amplifier circuits while providing excellent DC characteristics.
The demand for improved operational amplifiers, and in particular instrumentation amplifier circuits for high-precision data acquisition and instrumentation applications, such as multi-channel data acquisition systems, current shunt monitors, and industrial or physiological sensors, continues to increase. Instrumentation amplifier circuits are generally designed to amplify the difference between two voltage inputs with a defined gain, wherein a single-ended output is provided which is referenced to a known reference point, for example, ground.
There are a variety of instrumentation amplifier circuits available today. One conventional instrumentation amplifier comprises three op-amps as illustrated in FIG. 1. Buffers A1 and A2 are configured to receive a differential input voltage VIN, i.e., the difference between input voltages VIN+ and VINxe2x88x92, at the positive input terminals of amplifiers A1 and A2, and which are configured with resistors R1 through R3 to provide a buffered signal to a third amplifier A3. For example, the outputs of amplifiers A1 and A2 are applied as an input to a third op amp, A3, configured as a difference amplifier, and coupled to resistors R4-R7. As a result, the differential voltage can be buffered through amplifier A3 to the output terminal VOUT.
Another approach for an instrumentation amplifier with current mirrors is disclosed by Toumazou and Lidgey in xe2x80x9cNovel Current-Mode Instrumentation Amplifierxe2x80x9d, Electronics Letters, Vol. 25 No. 3, Feb. 2, 1989, and is illustrated in FIG. 2. Instrumentation amplifier 200 comprises a pair of current mirrors CM1 and CM2 which are configured to mirror the supply currents from both the op amp and the output stage circuit of unity gain buffer A1, the VIN+unity gain buffer. Again, only the difference in supply currents, VIN/RIN, flows out of the output of current mirrors CM1 and CM2 and into resistor ROUT. As a result, a voltage is developed equal to VIN X (ROUT/RIN) that can be buffered through unity gain buffer A3 to the output terminal VOUT.
While the above instrumentation amplifier configurations can provide good DC common mode rejection, such instrumentation amplifiers have great difficulty in providing a for good rail-to-rail voltage swing capability for input and output stages. While it would be highly desirable if these instrumentation amplifiers could provide rail-to-rail voltage swing capabilities, currently available instrumentation amplifiers cannot suitably provide true rail-to-rail voltage swing due to their operating characteristics.
Current mirrors, like those of FIG. 2, can affect the ability of amplifier circuits to realize optimum rail-to-rail output performance because those current mirrors require significant headroom. A current mirror typically allows the input current to flow through a gate-drain connected device, i.e., a diode-connected device. This input current results in a voltage from gate to source VGS that can be applied across a second device that replicates the input current. For example, with reference to FIG. 4, a current mirror 400 is illustrated and which comprises transistors M1 and M2, with transistor M1 configured in a diode-manner, i.e., with a gate-to-drain connection. Current mirror 400 is configured such that as a current I flows through transistor M1, a gate-source voltage VGS results that can be applied across transistor M2, thus replicating the current I to flow out of transistor M2 if the devices are perfectly matched.
To facilitate matching devices M1 and M2, it is desirable that the voltage at node A is equal to the voltage at node B. Thus, for applications with gate-source voltage VGS equal to one volt at node A to facilitate good matching or accurate current mirroring, for example, node B is also required to be equal to one volt. However, at a gate-source voltage VGS equal to one volt at transistor M1, forward-biasing of the source-to-substrate diode junctions within transistors M1 and M2 can occur when the substrate is grounded and when charge pumping the sources of transistors M1 and M2 negative. On the other hand, when charge pumping transistors M1 and M2 and the substrate negative, noise can be coupled back into the body of all the NMOS devices on the integrated circuit. Unfortunately, to minimize the effects of forward-biasing of the source-to-substrate diode junctions, a limited amount of negative charge pump voltage can be applied to the sources of transistors M1 and M2 which results in limited headroom being available. Moreover, to provide for an output of zero volts, e.g., to provide the output at ground, the sources of transistors M1 and M2 would operate at xe2x88x921 volt, again resulting in a forward-biasing of the substrate-to-source diode junction on the NMOS devices of current mirror 400.
Accordingly, a need exists for a circuit having a current mirror configuration that can enable an amplifier circuit to realize improved rail-to-rail swing capabilities. In addition, a need exists for a current mirror configuration capable of being implemented within a single-well process having limited voltage headroom requirements.
The method and circuit according to the present invention addresses many of the shortcomings of the prior art. In accordance with one aspect of the present invention, an improved current mirror configuration can be integrated in the output stage of an operational amplifier that enables improved rail-to-rail performance of the amplifier.
In accordance with an exemplary embodiment, an operational amplifier can be configured with a current mirror configured within the output stage of the operational amplifier. Further, through use of the current mirror configured in a feedback arrangement within the amplifier, the substrate of the integrated circuit can be suitably grounded to minimize noise problems. For example on a single n-well process, less drain-source VDS voltage on the input and output side of the integrated current mirror is required to obtain improved rail-to-rail output performance.
To facilitate the obtaining of rail-to-rail output performance, the operational amplifier can also incorporate a positive and a negative charge pump. However, instead of requiring the negative charge pump to charge pump the current mirror negative a full VGS voltage, the current mirror requires minimal headroom, e.g., approximately VDSAT, for implementation with the operational amplifier. For example, the integrated current mirror enables the charge pump to only charge pump the current mirror negative by approximately ⅓ VGS voltage so that the NMOS source-to-substrate junctions are not forward-biased to the grounded substrate. As a result, the current mirror requires less drain-source VDS voltage to obtain full rail-to-rail output performance while allowing the substrate to remain grounded such that noise from the charge pump, which might otherwise charge pump the sources of the current mirror as well as the substrate, will not couple into other NMOS devices or the integrated circuit through a back body effect.
In accordance with another exemplary embodiment, the current mirror can be configured with chopper stabilization to facilitate reduction of mismatch errors within the transistor devices of the current mirror.